There is currently a large and growing requirement of nitrogen trifluoride (NF3) for use in semiconductor manufacturing. Nitrogen trifluoride may be used, for example, as an etchant or chamber cleaning gas. On an industrial scale, NF3 is typically manufactured by the fluorination of an ammonium bifluoride/HF complex. There are two principle methods for fluorination of this complex: direct fluorination (DF) and electrochemical fluorination (ECF).
In the direct fluorination of NH3 or NH4+ salts to produce NF3, there are competing reactions such as the following:3 F2+NH3→NF3+3 HF  (1)3 F2+2 NH3→N2+6 HF  (2)4 F2+2 NH3→N2F2+6 HF  (3)The most favored reaction according to thermodynamic calculations is (2), which produces only undesirable N2 and HF. The prior art has attempted to enhance reaction (1) to produce NF3 and minimize the extent of reactions (2) and (3).
The prior art provides a variety of direct fluorination methods for the synthesis of nitrogen trifluoride. For example, U.S. Pat. No. 4,091,081 describes a direct fluorination method for manufacturing nitrogen trifluoride whereby gaseous F2 is contacted with liquid (molten) ammonium acid fluoride (AAF) while gaseous NH3 is separately contacted with the liquid AAF to generate ammonium ions. The '081 process typically provides NF3 yields of 40–50%. It is operated to maintain a molar ratio of by-product HF to ammonia, referred to as the melt ratio, of 2.0 to 2.5 in the reaction liquid and at temperatures above the melting point of ammonium bifluoride, NH4HF2, which is 127° C. The contacting of F2 with AAF is done using a specially designed sparger having a plurality of small holes.
U.S. Pat. No. 5,637,285 describes a DF method for production of nitrogen trifluoride by the fluorination of an ammonium acid fluoride, NH4H(x−1)Fx where x is at least 2.55 and the melt ratio of HF to ammonia is at least 2.55. Unlike the '081 patent, the reaction is conducted in a stirred reactor. The '285 process generally produces higher NF3 yields than the '081 process.
U.S. Pat. No. 6,183,713 describes a method for generating nitrogen trifluoride by the fluorination of a packed bed containing a particulate ammonium complex of a metal fluoride. The ammonium complex within the bed needs to be continuously replenished in order to sustain the reaction.
Published Application WO 01/85603 A2 describes a process for generating nitrogen trifluoride through the reaction of ammonia and fluorine in the gas phase. The reaction is performed at temperatures of 80° C. or less in the presence of a diluting gas such as nitrogen, helium, argon, hexafluoroethane, and octafluoropropane.
Japanese Patent Application No. 1-307243 describes a process for generating nitrogen trifluoride through the reaction of gaseous ammonia and gaseous fluorine wherein either gas is first dissolved in a perfluorocarbon fluid. The reaction occurs in a non-stirred column at a temperature that ranges from −30° C. to the boiling point of the solvent. The perfluorocarbon fluid acts as a heat sink because the reaction between the NH3 and F2 gases is highly exothermic.
The direct fluorination method, while commonly used, has several drawbacks. First, the utilization of fluorine delivered to the process is less than 100%. Consequently, a significant amount of the fluorine is not reacted and must be scrubbed. Fluorine, unfortunately, is expensive to produce and difficult to handle. Thus, a utilization of less than 100% of the fluorine within the process is economically disadvantageous. Second, the process of scrubbing unreacted fluorine generates unwanted toxic by-products such as OF2. These by-products can be difficult to separate from NF3 and create excess waste to be disposed. Third, the reaction medium may be highly corrosive to metal reactors and components at typical operating temperatures. Because of this, the reactors and components may eventually become contaminated by the dissolved metal ions within the reaction medium thereby leading to the disposal of the reaction medium. Lastly, the reaction between fluorine and ammonium ion may not be 100% selective to the formation of NF3 and varying amounts of by-products, e.g., N2 and N2F2, may be formed.
There is a need in the art for a nitrogen trifluoride production method that is more selective in the formation of nitrogen trifluoride rather than N2 or N2F2. There is an additional need in the art for a method that reduces the corrosion of the metal reactors and related components during nitrogen trifluoride production. Further, there is a need in the art for a method that facilitates the removal of the HF by-product during the synthesis of nitrogen trifluoride. Yet another need in the art is a cost-effective method for nitrogen trifluoride production that increases the conversion of F2.
All references cited herein are incorporated herein by reference in their entirety.